Heterogeneous photocatalysis has shown promise as a chemical method for oxidizing and thereby removing unwanted organic compounds from fluids, including water, and air. A UV-illuminated catalyst, such as titanium dioxide, absorbs UV light, which produces electrons and holes that migrate to the surface of the catalyst. At the surface, the electrons reduce adsorbed oxygen, while the holes oxidize organic compounds or adsorbed water molecules.
For example, titanium dioxide is a semiconductor with a band gap of 3.0 eV (rutile) and 3.2 eV (anatase). When a photon having an energy in excess of the band gap is absorbed by the pigment particle, an electron is promoted from the valence band to the conduction band. The promotion of the electron produces a "hole." The hole and the electron may diffuse to the surface of the pigment particle where each may chemically react. Surface electrons generally reduce adsorbed oxygen, while surface holes generally oxidize organic compounds or adsorbed water molecules. When electron vacancies (holes) react with water, reactive OH radicals and protons are formed.
In related U.S. Pat. No. 5,256,616 and related U.S. patent application Ser. No. 08/244,149, ceramic beads coated with titanium dioxide efficiently induced the oxidation of oil on water when exposed to natural sunlight. Others have also utilized photocatalytic reactions to remove contaminants from air and water. See, for example, U.S. Pat. No. 5,045,288 to Raupp et.al.; PCT Patent Application No. PCT/US90/07651 to Lichtin et al. published Jul. 11, 1991; Japanese Kokai Patent Application No. Sho63[1988]-100042 to Kume, published May 2, 1988; and Australian Patent Application No. PH7074, Jul. 22, 1987 of Matthews.
While the use of photocatalysts for the removal of organic pollutants is generally known, a commercially feasible process for the use of such catalysts has not been developed. Known methods for adhering the photocatalyst to a reactive surface such as heat sintering have been impractical or too expensive to apply to very large surface areas such as those for air cleaning or the production of large areas of self-cleaning surfaces. In a fluid reactor, e.g., for air or water, a large photocatalytic surface area in contact with the air or water to be cleaned is advantageous. In prior art methods of bonding the photocatalyst to a surface, such as sintering, the actual catalyst surface area per unit geometric area was reduced. Consequently, reactors functioning at a given flow rate, required large catalyst-coated surface areas. This not only increased the required size of the reactor, but also created a large pressure drop in the reactor, making the reactor expensive to operate. Because rates of removal of contaminants from air or water in a reactor have been too low to justify the high costs involved and photocatalytic air or water cleaning and reactors have not been introduced for general use, i.e., for cleaning air in office buildings, homes, restaurants or factories, or for cleaning water in homes or municipal water plants.
The photocatalyst compositions of the prior art are not easily applied to substrates; and especially to those having a large geometric surface area such as walls, windows and the like. Typically, such compositions are generally applied as an aqueous slurry, and are poured, sprayed, dipped, or otherwise coated onto a substrate as a thin film. The photocatalyst is adhered to the substrate by heat sintering or calcifying. To gain any thickness of the photocatalyst film, the process must be repeated many times.
When pigments such as titanium oxide (TiO.sub.2) are used in paints, hydrocarbon binders such as latex or polyurethane are generally used to adhere the pigment particles to each other and to a surface. Since photocatalytic activity of the pigment particles has been considered undesirable in paint, e.g., due to destruction of the hydrocarbon binders and the like, considerable effort is made to prevent it. This is because the oxidative reactions of the photocatalyst in the presence of sunlight and air degrade the polymeric hydrocarbon binder and causes undesirable "chalking" of the paint.
To-mask the activities of the photocatalyst, the TiO.sub.2 pigment particles in paints are generally enveloped or coated with a nonporous composition containing aluminum oxide (Al.sub.2 O.sub.3), silicon dioxide (SiO.sub.2), or both. These include, for example, TiO.sub.2 pigments available from Kronos, Inc., Houston, Tex. as catalog numbers 1000, 1070, 1072, and 1074; Lansco 8042 from Landers Segal Color Co., Passaic, N.J. This overcoating prevents diffusion of electrons or holes to the pigment surface thereby preventing contact and reaction with the polymeric binder, e.g., latex or polyurethane, or with water or oxygen. Essentially all of the Tio.sub.2 pigment used today in the manufacture of paints is rendered less photoactive by overcoating with aluminum oxide, silicon dioxide, or both. While this overcoating permits application of the pigment to a surface and preserves the appearance of a painted surface, it precludes photocatalytic reactions desired to remove contaminants from a surface, air or water.
To solve these problems and to produce a photocatalyst-binder composition useful in a variety of applications, it has surprisingly been found that photocatalyst particles adhere to a substrate by the use of substantially non-oxidizable binders. Being permeable to the organic contaminant-reactant and to oxygen, as well as their reaction products, carbon dioxide and water, such binders do not impede, and can even enhance, photocatalytic activity.